A field-effect transistor (FET) in the form of a thin film is composed of a source electrode, a drain electrode, a gate electrode, an insulating layer, and a semiconductor layer, and recently, interests in an organic transistor in which an organic material such as a single molecule, a polymer, and an oligomer is applied to a semiconductor layer have been increased.
Among the organic materials, a single molecule for a solution process can be applied to a flexible substrate, can be subjected to a low temperature process, and can be applied to a large area, thereby improving processability and economic feasibility. Further, the single molecule for a solution process has no batch-to-batch difference as compared to a polymer and thus is advantageous in commercialization when applied to a semiconductor layer.
Meanwhile, the structure of the organic transistor may have a top gate or bottom gate structure depending on the position of a gate electrode, and the bottom gate structure may be divided into a top contact or bottom contact structure depending on whether the source/drain electrode is disposed on or below the semiconductor layer.
The top gate structure may be advantageous in performance because the semiconductor layer has a relatively large area in which the semiconductor layer is brought into contact with the source electrode and the drain electrode, and the top gate structure may be advantageous in air stability because a top electrode is applied onto the semiconductor layer. In contrast, the bottom gate structure may be disadvantageous in stability as compared to the top gate structure.
The performance of an organic transistor may be evaluated in terms of charge mobility, on/off ratio, and the like, and a high performance organic semiconductor needs to be developed in order to improve the performance of the organic transistor.
Meanwhile, a gas sensor is installed in various places and thus serves an important role to monitor hazardous materials and contaminants in the atmosphere and environment where we live, and requires characteristics such as a quick response showing how fast the gas sensor can respond to a situation, sensibility showing that the gas sensor can respond to the detection of gas even when detecting a small amount of the gas, durability showing how long the gas sensor can operate, and economic feasibility showing that the gas sensor can be used by consumers without burdens. Further, the gas sensor needs to have characteristics of the easy integration and enumeration in order to be combined with an existing semiconductor process technology.
Examples of the operation principle of the gas sensor include a semiconductor type which uses a change of resistance values according to a change in amount of gas and an oscillator type which uses a change in frequency when gas is adsorbed onto an oscillator, which oscillates with a predetermined frequency. Most of the gas sensors have been used as the semiconductor type gas sensors having simple circuits and showing stable thermal characteristics at room temperature.
The semiconductor-type gas sensor is divided to an inorganic semiconductor-type gas sensor in which a silicon semiconductor being an inorganic material forms semiconductor crystals through interatomic covalent bonds and an organic semiconductor-type gas sensor in which the semiconductor crystals are bonded to each other by molecular bonds of a conductive polymer, that is, van der Waals interaction. Among them, a gas sensor based on tin oxide requires a high temperature to operate the sensor, so that products having a micro heater therein are commercially available, and it is difficult to manufacture the gas sensor into a thin film due to the problem. Further, the gas sensor has a limitation for being utilized over a wide application because it is impossible to measure a gas concentration with sensitivity of 50 ppm or less.
A gas sensor based on an organic semiconductor may significantly lower costs for manufacturing an existing gas sensor because the gas sensor can be manufactured in a solution phase through various printing processes by dissolving a semiconductor material in an organic solvent. Therefore, recently, studies have been actively conducted on a printing-type gas sensor which reports an organic semiconductor as a sensing material through a printing process such as inkjet printing, but there have been relatively few research studies conducted on various organic semiconductor materials for sensing.